The present invention relates to various methods and apparatus for testing materials, machinery, and so forth. More particularly it relates to non-destructive testing techniques, and especially to non-destructive testing in which acoustic emissions of bodies under test are detected and analyzed.
Detection and analysis of acoustic emissions have become an important tool in non-destructive testing. A respectable body of knowledge, expertise, and methodology has developed in this area due to the very important advantages which obtain in certain testing situations. U.S. Pat. No. 3,985,024, issued Oct. 12, 1976 (Horak), for example, discloses three such acoustic emission processes. More particularly, the patent is directed to an acoustic emission process which rejects unwanted signals by means of three methods used either independently or in conjunction. In the first, a master-slave arrangement is used in which any signal received by an external slave transducer before it is received by a master transducer is rejected. Second, rise time discrimination, which allows only signals which satisfy certain wave form requirements to be counted, is employed. The third method used discriminates among signals received by the master transducers on the basis of whether they occur coincidently within a selected time frame.
Similarly, U.S. Pat. No. 3,858,439, issued Jan. 7, 1975 (Nakamura), is directed to a master-slave arrangement in which signals are accepted only if they reach the master sensors before they reach the slave sensors. Frequency filters are also provided to exclude signals outside the desired frequency range, thereby excluding background noise.
U.S. Pat. No. 3,919,883, issued Nov. 18, 1975 (Nakamura), monitors the rate of amplitude build-up of the acoustic signals, and selects only acoustic signals which have a selected build-up rate.
Similar methods for analyzing acoustic patterns have been applied, for example, to detecting glass breakage. In U.S. Pat. No. 4,091,660, issued May 30, 1978 (Yanagi), for example, there is disclosed a method for detecting breaking glass by identifying the simultaneous production of a low frequency (&lt;50 kHz) component and a high frequency (&gt;100 kHz) component. The output of a piezoelectric element mounted on a glass plate is first passed through a low pass filter and a parallel high frequency resonant circuit. The two signals are then applied to a gate circuit so that an output signal is produced only when the levels of both frequency components are above their respective predetermined levels.
U.S. Pat. No. 4,134,109, issued Jan. 9, 1979 (McCormick et al), discloses an alarm system responsive to the breaking of glass. The system compensates for the level of background noise by analyzing acoustic signal strength, frequency content, and signal pattern. In one part of the system the signal is supplied to narrow bandpass filters tuned to pass four different acoustic frequencies. Each filter output is supplied to a corresponding waveform integrator and comparator so that output signals are generated only when the corresponding filter signal exceeds a threshold level. A further output signal is generated only when the majority of the frequency bands exceeds the threshold level.
U.S. Pat. No. 3,614,724, issued Oct. 19, 1971 (Brown et al), discloses a means for detecting man-associated disturbances and distinguishing them from background noise by passing seismic signals through a series of frequency band pass filter-amplitude detector combinations connected in parallel. The detector outputs are processed so that signal pulses are generated only when the amplitude of the input to one pass band is greater than a certain level above that of the other pass bands.
U.S. Pat. No. 4,317,368, issued Mar. 2, 1982 (McElroy), discloses an acoustic emission warning system which detects glass fiber breakage in fiberglass members. Acoustic signals are first amplified and filtered by one network, and then rectified and demodulated to produce a representative envelope. These envelopes are then passed in parallel through high and low threshold stages, each comprising a comparator amplifier which receives a constant preset voltage tapped from a potentiometer. The threshold stage outputs are then sent through a three part logic network which identifies the wave envelope. If the envelope corresponds to glass fiber breakage it is counted. An alarm is activated if the count frequency exceeds a preset level.
Clearly, a variety of non-destructive acoustic emission testing methods and techniques, as well as acoustical analysis methods for other applications, is thus present in the literature. However, none of these publications provides a direct adjunct facility which can generate, as desired, companion data furnishing accept or reject criteria for deciding, on a reported-event by reported-event basis, the validity of each particular event as it is accepted as such by an existing, "standard" acoustic emission detector. (By "standard" is meant a conventional, "off-the-shelf", state-of-the-art detector, such as a Dunegan/Endevco (D/E) Basic Acoustic Emission Instrument with Model No. 1801-110B Preamplifiers and Model No. 1801-50H Frequency Discriminator Preamplifiers, available from Physical Acoustics/Dunegan Corp.) A need therefore remains for such an adjunct event discriminator for further refining and characterizing the data generated by such standard acoustic emission detectors.
Even more generally, as discussed further below, it has been discovered that in acoustic emission non-destructive testing (as distinguished from target destructive events such as glass breakage), there can be important frequency-dependent means for identifying valid events which are indicative of structural flaws in the material being examined, and distinguishing these from mere noise. This is an important distinction from the above-noted patents which use frequency discrimination techniques, for example, for detecting actual breakage of glass materials.
A specific example of a particularly effective application of such a technique may be found in the testing of the thermal protection system tiles of the Space Shuttle. Usually the transducer and a frequency bandpass are selected to detect signals in a frequency range such as 80-160 kHz and reject others containing energy only outside this range. This is a known prior art technique. However, when the tiles are being tested, there are other possible sources of noise which are broadband in frequency, and which can be generated by (1) tile rubbing on an adjacent tile, (2) grit under the vacuum chuck, (3) vacuum leaks, or (4) rubbing of the tile coating crack faces. The novel frequency discriminator according to the present invention provides a method and apparatus for determining if these other types of broadband extraneous noises occur during a test.
Of course, it will also become apparent that valuable applications of the present invention can be made in many other acoustic emission non-destructive testing applications as well.
A need therefore also remains for a frequency-dependent means for identifying valid acoustic emission events during non-destructive testing which are in fact indicative of structural flaws in the material being examined, and for distinguishing these from mere noise.